Enantioselective Cyanosilylation of Ketones Catalyzed by a Chiral Oxazaborolidinium Ion

The chiral oxazaborolidinium salt 1 (X = TfO) is an excellent catalyst for the cyanosilylation of methyl ketones promoted by trimethylsilyl cyanide and diphenylmethyl phosphine oxide as co-reactants (to generate Ph(2)MePOTMS(N=C:) as a reactive intermediate). The face selectivity of this reaction parallels that previously observed for the corresponding reaction of aldehydes. A unifying and rational mechanistic explanation is provided for these enantioselective reactions. Evidence is presented to support the importance of alpha-C-H...O hydrogen bonding, pi,pi-interaction of the complexed ketonic carbonyl with the mexyl group of 1, and an early transition state for high enantioselectivity. The cyanosilylation reaction described herein provides access to many useful chiral compounds.